CMOS IC design of broadband communications components operating at rates of 10 Gb/s and higher are now commonplace and have reached a level of maturity in recent years. However, circuits running at these speeds continue to pose interesting design challenges as the specifications for these circuits become more stringent.
At the heart of any serializer/deserializer chip is the internal high-speed clock that generates the synchronization signal—generally a voltage-controlled oscillator (VCO). In order to maintain proper signal integrity it is imperative that the VCO output exhibit sufficiently low jitter. This jitter arises from both internal noise generation (e.g. random jitter) and disturbances from outside the VCO circuitry (e.g. periodic jitter). Techniques for reducing the random jitter generated by the VCO itself are well known; however, predicting and guarding against external disturbances is a more difficult task. This is exacerbated by the trend to place more circuitry—particularly digital blocks—on a single chip (e.g., SOC), thereby coupling in significant switching noise. The effect of much of the noise coupling from the substrate can be reduced by appropriate layout techniques (e.g. guardbands or use of additional wells), however it is more difficult to reduce the effect of noise coupled directly from the power supply thus making a VCO with high PSRR (power supply rejection ratio) desirable.
Voltage-controlled oscillators (VCO) tend to be sensitive to perturbations in the supply voltage. Such perturbations can cause excess jitter in the VCO output, which increase the bit error-rate of the communication system in which it is implemented.
Using differential signaling techniques (i.e., current mode logic or CML topology) can somewhat weaken this effect.
Attempts to address the sensitivity of VCOs to such perturbations in supply voltage have included employing an additional, dedicated power supply that provides a very clean supply voltage and by adding extra regulator circuitry, such as a low drop-out regulator, which consumes considerable extra power.